A solar cell is a device able to convert incident light to electrical power. Many solar cells are typically grouped to form an array of solar cells. To collect the electrical power from the solar cells, groups of cells are either directly connected in series or in parallel. Where the cells are connected in series, they must have identical currents but if the cells are connected in parallel they must operate with identical voltages. An individual cell will produce maximum power at a unique cell voltage and current which will vary from cell to cell. The combination of voltage and current that allows a cell to produce its maximum power is termed the maximum power point. The maximum power point varies with cell illumination and temperature. Connection of the cells in series forces cells to have identical current while connection in parallel forces cells to have identical voltage. Direct connection in series or parallel results in failure to collect all the available electrical power from the solar cells in the array and at least some of the cells will operate at a condition other than at their maximum power points.
To obtain the maximum available power from a group of solar cells connected in an array or sub-array, a maximum power tracking device is used. Maximum power tracking devices are DC to DC power converters that allow an array or sub-array to operate at their maximum power point. A DC to DC converter can transform a power input at a certain voltage and current to be transformed to a DC power output at a differing voltage and current. A key feature of all maximum power trackers is a control device that determines the point of maximum power for the connected solar cells and acts to adjust the DC to DC converter performance to adjust the cell voltage or current to extract the maximum available power.
However there are a number of problems or disadvantages associated with the use of a single maximum power device to control the voltage or current of an array or sub-array of solar cells.
Where solar cells are used to power vehicles, the vehicles are usually aerodynamically designed with curved surfaces and also have limited surface area in which to mount the solar cells. Consequently arrays of cells are mounted on the curved surfaces but the variation of the angle of incidence of light on the different cells within the array on the curved surface causes variation in the available optical power. Furthermore, cells in an array may be subjected to variable light levels due to shadowing by foreign objects such as trees and buildings between the cell and the source of illumination.
Because of differences in optical illumination, cell temperatures may vary within arrays causing some cells to be hotter than other cells. Arrays may be cooled partially by air flow or by the use of a cooling fluid in an illumination concentrator system. These mechanisms however may not provide uniform cooling to all cells.
The available power from each cell within an array will vary due to the variations in illumination and temperature. In these cases, the maximum power conditions of different cells within the array will differ at any one point of time. Furthermore the maximum power conditions of some cells within the array will vary differently over time compared with others. As well these variations are not predictable. In addition changes to the maximum power conditions of cells can vary rapidly thereby requiring a relatively quick response time.
Currently maximum power tracking devices are directly electrically connected to an array of solar cells. A single maximum power tracking device is currently used to control the available power from an array of between ten to several hundred cells.